Miss at cruise !!!

Alright, here is the senario, Driving, starting out I will get a mild miss int he engine, hard acceleration, accelerates fine, no miss. Once at cruising speed then it starts missing again( no matter what cruise speed) If I move the accelerator a little bit it will stop and accelerate, till once again its a a cruise then it will miss. I have replaced the Plugs,wires,cap, rotor and fuel filter. I have tried to isolate by plugging off the EGR valve but the problem is still there. ( disconnect vacuumline) Engine is a 89' 4.0 w/ auto tranny.
I am thinking maybe the TPS may be bad, but I am leary at throwing parts at it. It also has 149K, could the timing chain be the culprit.( Slack in the chain)
The odd thing is that it does it only in gear, will rev up fine in Park, and even with the tanny in gear and brakes on, it will idle up smoothly. :banghead:

My first thought is the TPS. But it could be a bad plug wire also. Sometimes an older vehicle can be sensitive to the routing of the wires, so make sure they're not crossed at any point.

Fred,

I replaced all the wires, thinking that same thing but its still there. I am stuck here. I might try the TPS, but I think it might be something else, only problem is what!

Is it a consistent miss, at regular intervals, or is it at odd/random intervals?

If it's consistent, I'd say electrical to a cylinder, or the cylinder itself.

But random...I'm thinking way back now, but the Chrysler 5.2L (when it was still the 318cid) would develop a miss in the conditions you're stating when the timing chain got stretched - so we're in the same ballpark on that.

But still, I'd try the TPS first. Try this experiment: figure out where the throttle is at cruise, when the problem shows itself. Then, find a place where you can try it out from a dead stop, from 10mph, 20, etc and aim to have your foot/throttle at the same place. I'd bet that if it's the TPS you'll find the miss to be at the same throttle position, no matter the speed.

Thanks Fred,
I priced the TPS, OUCH! Around $70 (US) ! So I just might have to take the plunge and get it. I will check the "backlash" of the timing chain firstthough, I believe the rule of thumb was any more than 3 degrees deflection, replace it.

Sounds like a worn spot on the TPS.

A lot of votes for the TPS...I would look into that myself.

Best regards...

I had the the same problem just replaced the tps sensor on my 89 cherokee and it runs fine now cost me $160 cdn

Thanks everyone, I guess the verdict is the TPS sensor. I will get it as soon as I 'm able and let you all know how it turned out.

Ok, I have the TPS, now as far as installingit? I see that the TPS is adjustable, so once I remove the old TPS unit how do I properly adjust the new unit? Or do i do my best to install it in the same position as the old one?

Don't have the exact information...for the 89 MY?

The information that I have is for one with four wires in a square plug.
Letters are molded in the plug; A-D.
D is for the negative on the voltmeter (hope you have a digital one).
Test A to D should be input voltage 5.0 Volts.
Test B to D should be about 4.2 volts for the throttle plates closed.

The later one is a three wire and off idle is 0.06 volts. 5 volts feed.

Maybe you can just test the old one at idle for the voltage.

Let me know which one it is you have.

I actually have 2 connectors, flat-4 and square plug. So what your saying is with the throttle plate closed, those are the readings I should get if I have the TPS adjusted right?

I think you might have the four (which is square).
Check this link out:
http://www.lunghd.com/Downloads_and_Links/Downloads.htm
He has a download near the bottom for TPS Renix style.

However, as I mentioned if you took voltmeter readings from the original, you should have:
input which is always 5.0 V and the output from the one that you have (at idle)... you are going to be in the right ball game. Set with throttle plate closed.

With a good Digital Volt Ohm Meter (such as a Fluke DVOm) it doesn't matter which is positive or negative. You might need paper clips to probe the connectors.

Ok back to the drawing board! Installed and set up the TPS, No luck. The "miss" is still there. I cleaned the throttle body and throttle plate, runns a little stronger but it still misses, pops and jerks at a cruise( no load) while driving. You can rev the engine up all day long and it will tach up fine, ( out of gear) , place it in gear and tromp the gas and you will feel a mild "miss" at WOT. I think I'll check the fuel pressure and even unblot the fuel rail and see if I might have a crudded up injector(s). I am thinking that the problem might lie in the fuel system. ( I have run almost 2 tanks of premuim gas) Vehicle sat for a long while too so maybe the injector or even a loose distributor shaft. ( 148K)

Oh, Thanks Aloha for the pdf, it helped allot with setting up the TPS!

Have you ever heard of this?

http://www.angelfire.com/my/fan/MAP_adjuster.html

You could use this for "testing". At the very least...test for enough voltage at the MAP sensor (5.0 volts).